1. Field of the Invention
This invention relates to prosthetic implants and more particularly to a prosthetic wrist implant with a geometry that closely matches that of the natural wrist and which minimizes bone resection. The invention also includes methods and instruments used to perform the implantation of the prosthesis.
2. Description of the Related Art
The replacement of degenerated natural joints with man made prosthetic replacements is well known, including the replacement of the wrist joint. Prior to the introduction of prosthetic joint replacement, patients with wrists diseases such as radio-carpal arthritis were often treated with a fusion procedure. Fusion involves an implantable joint replacement that prevents articulation of the wrist. Fusion however, was less than satisfactory since it left the patient with no movement in the wrist.
Existing prosthetic wrist implants have a number of drawbacks. Because of the size needed to achieve the necessary strength, wrist implants have typically required excessive amounts of bone to be resected. For example, in these implants relatively large and lengthy implant stems were inserted into the radius and carpal bones. This significantly weakened the bones making them more susceptible to post operative fracture. Furthermore, once fracture occurs, because of the significant loss of bone due to resection, their may not be enough bone left to permit a satisfactory fusion procedure. This may leave a patient without any avenue of treatment, not even fusion.
Another drawback with prior prosthetic wrist implants is that they do not provide the patient with acceptable functionality. Part of the problem is that these implants have not adequately matched the geometry of the natural wrist. As a result, flexion and extension of the hand, return to a natural position, and range of motion have all been less than optimum, and the patient is left with less functionality of the implanted wrists than with a natural wrist.
Another difficulty with prior wrist implants involves the means of attaching the implant to the radius and carpal bones. If the attachment is not secure enough, or the implant itself is not strong enough, the stability of the attachment may not be adequate under normal use of the wrist. Because of this, larger implants and implant stems have been used to attach the implant components into the bone. However, as discussed above, this results in more resection and weakening of the bone. As a result of the above issues, existing prosthetic wrist implants have not always achieved adequate levels of patient satisfaction and have sometimes resulted in unacceptable complication rates.
Another problem with wrist implant procedures is the unavailability of special instruments for performing the implant operations. Tools currently available make excavation of a hole, which is cut into the bone to position the implant, difficult and imprecise. As a result, malrotation of the tool may occur which results in misalignment of the implant. Further, specialty templates are unavailable to assist in alignment of a cutting blade during resection of the bone into which the implant is to be fitted. The absence of a template during the actual resection requires the surgeon performing the resection to "eyeball" the area to be resected. This may result in excess bone material being removed during the resection.
Thus, it would be desirable to provide an improved prosthetic wrist implant, method, and instruments which overcome some or all of the above-discussed problems. In particular it would be desirable to provide a wrist implant which has a geometry which matches that of a natural wrist and which affords the patient a natural range of motion, natural flexion and natural extension of the hand. Furthermore, it would be desirable to provide a prosthetic wrist implant which is small enough to minimize the bone resection required. It would also be desirable to provide an improved method for attaching a prosthetic wrist implant which provides a stable and strong attachment to the bone without requiring excessive loss of bone through resection or drilling. In addition it would be desirable to provide instruments for implanting a prosthetic wrist implant which minimizes bone loss from resection and which affords the patient the above-described desirable features.